Quercetin-metabolizing CYP6AS Enzymes of the Pollinator Apis Mellifera (Hymenoptera: Apidae)

Although the honey bee (Apis mellifera) genome contains far fewer cytochrome P450 genes associated with xenobiotic metabolism than other insect genomes sequenced to date, the CYP6AS subfamily, apparently unique to hymenopterans, has undergone an expansion relative to the genome of the jewel wasp (Nasonia vitripennis). The relative dominance of this family in the honey bee genome is suggestive of a role in processing phytochemicals encountered by honey bees in their relatively unusual diet of honey (comprising concentrated processed nectar of many plant species) and bee bread (a mixture of honey and pollen from many plant species). In this study, quercetin was initially suggested as a shared substrate for CYP6AS1, CYP6AS3, and CYP6AS4, by its presence in honey, extracts of which induce transcription of these three genes, and by in silico substrate predictions based on a molecular model of CYP6AS3. Biochemical assays with heterologously expressed CYP6AS1, CYP6AS3, CYP6AS4 and CYP6AS10 enzymes subsequently confirmed their activity toward this substrate. CYP6AS1, CYP6AS3, CYP6AS4 and CYP6AS10 metabolize quercetin at rates of 0.5+/-0.1, 0.5+/-0.1, 0.2+/-0.1, and 0.2+/-0.1 pmol quercetin/ pmol P450/min, respectively. Substrate dockings and sequence alignments revealed that the positively charged amino acids His107 and Lys217 and the carbonyl group of the backbone between Leu302 and Ala303 are essential for quercetin orientation in the CYP6AS3 catalytic site and its efficient metabolism. Multiple replacements in the catalytic site of CYP6AS4 and CYP6AS10 and repositioning of the quercetin molecule likely account for the lower metabolic activities of CYP6AS4 and CYP6AS10 compared to CYP6AS1 and CYP6AS3.